I managed to install them now and they seem to be running too.
I deinstalled everything (once again), deleted everything (also once again) loaded 0.7.45, installed this version. Installed all plugins (even the new plugin updater), updated Calibre to 0.7.50 and now all plugins are running. Even the Kindle Collections plugin.

Does the plugin updater use another routine? If yes the Calibre routine is buggy, if no I don't know what is different, as with the plugin updater installing of the "Search the internet" plugin runs like a charm, as I had to uninstall it because it wasn't functioning.